Various ophthalmologic apparatuses using optical apparatuses are used at present.
Among them, an optical coherence tomography (Optical Coherence Tomography: hereinafter, described as OCT) can acquire a tomogram of an eyeground to a depth of several mms with a spatial resolution of the order of micrometers.
Therefore, OCT gains in importance as a diagnostic tool which gives information that cannot be obtained with a conventional scanning laser opthalmoscope (SLO).
Conventionally, as OCT, TD-OCT (Time Domain OCT) has been known as disclosed in M. Blezinski, “Optical Coherence Tomography” Wiley, London (2006).
The TD-OCT is configured to measure a coherent light with a backscattering light of a signal arm and obtain information of a depth profile, by combining a broadband light source and a Michelson interferometer and scanning delay of a reference arm.
However, with such TD-OCT, a mechanical structure is required for scanning the delay in a wide range, resulting in difficulties in realizing high-speed image acquisition.
For the purpose to overcome above difficulties, SD-OCT (Spectral Domain OCT) according to a method of measuring spectral interference has been developed.
Further, SS-OCT (Swept Source OCT) according to a method of measuring spectral interference with a single channel optical detector by using a high-speed wavelength swept laser as a light source has also been developed.
In the SS-OCT, the speed of image acquisition is essentially determined by the wavelength swept rate of the high speed wavelength swept laser.
Accordingly, a high speed wavelength swept laser is enhanced in speed, which has led to a development of a mode-locking method named a Fourier domain mode locking.
By further improving the method, a swept rate of ˜200 kHz has been achieved, and a frame rate of ˜900 Hz and a volume rate of 3.5 Hz have been realized (see R. Huber, et al. Opt. Exp. Vol. 14, pp. 3225 (2006)).